1. Field of the Invention
The present invention relates to a method utilized in a wireless communication system and communication device thereof, and more particularly, to a method of handling radio link failure detection in a wireless communication system and related communication device.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, so that the system structure becomes simple.
In the LTE system, radio link failure (RLF) between a user equipment (UE) and a UTRAN (Universal Terrestrial Radio Access Network) occurs due to all kinds of signal impairments in a wireless environment, unfavorable signal propagation conditions, or even system malfunctions. Therefore, the UE configured with a component carrier or connected to a cell defines a certain timer for RLF detection. For example, when the UE detects a radio problem on a component carrier or on a cell, a timer will be started for detecting radio link failure of the component carrier or the cell, and when the timer expires, the UE considers that the radio link of the component carrier or the cell have failed. That is, the timer is used for determining whether radio link of the component carrier or the cell is failed when the radio problem occurs.
Toward advanced high-speed wireless communication system, such as transmitting data in a higher peak data rate, LTE-Advanced is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of LTE system. LTE-Advanced targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), multiple input multiple output (MIMO), etc.
For bandwidth extension, carrier aggregation is introduced to the LTE-Advanced for extension to wider bandwidth, where two or more component carriers are aggregated, for supporting wider transmission bandwidths e.g. up to 100 MHz and for spectrum aggregation. According to carrier aggregation capability, multiple component carriers are aggregated into overall wider bandwidth, wherein UE can establish multiple links corresponding to the multiple component carriers for simultaneously receiving and/or transmitting on each component carrier.
In addition, COMP is considered for LTE-Advanced as a tool to improve coverage of high data rates, cell edge throughput, and system efficiency, which implies dynamic coordination among multiple geographically separated points. That is, when an UE is in a cell-edge region, it is able to receive signal from multiple cells and transmission of the UE can be received at the multiple cells.
According to structure of the LTE system, each UE is only allowed to connect to a single cell with a single component carrier. When a radio problem is detected on the single component carrier or the single cell, the UE starts a timer for RLF detection only on the single component carrier or the single cell. However, according to structure of the LTE Advanced system, the UE operates with carrier aggregation, COMP or both, and connects to multiple cells with same component carrier or with multiple component carriers. The LTE Advanced system does not clearly specify how the RLF detection is applied for the UE with multiple component carriers or multiple cells. Therefore, usage of timer for the RLF detection in the multiple component carriers or multiple cells is never concerned. Improper timer initiation in multiple component carriers or multiple cells may cause abnormal connection malfunction of the UE.